A lot of cosmetics and shifting buttons from here to there, [...]
Hi @cubot! As you know, we have millions of users on the stable product. The beta user group is pretty large as well, but in order to deliver a good stable product, we need to run different experiments in the beta. Sometimes it's about shifting button locations, and sometimes it's about testing a completely new feature or measuring network performance. You will often face incomplete translations, and more bugs than the stable product.
On the positive side, you will always be running the latest version of Opera Mini, when using the beta. As @mbaluta said, if you prefer a more stable product, we'd be happy to have you as a user on the (non-beta) Opera Mini instead.